Great question — answering as the UphillTech team:
Both. It’s just a matter of taste and convenience what you prefer. The long-soak gives an overall softer, mellower result and requires a minimum of planning. The overnight-soaked version may be more digestible, but this largely personal, not necessarily a general rule. It’s also close to the “original” version. The pour-and-eat method preserves the flavors of the individual ingredients and retains the “crunch” of some of the components. It’s also more convenient for packed meals, if the liquid component is added at a later time - not refrigeration necessary (except for the milk or yogurt, obviously). You may even try out muesli cooked by a porridge method, but of course you’d be going...
Full answer by Stephie: https://cooking.stackexchange.com/a/96437 (CC BY-SA 4.0).
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